Neural tube defects

Neural tube defect (NTD) refers to the incomplete closure of the neural tube in very early pregnancy, resulting in cranial defects or spinal dysraphism.

The neural tube comprises a bundle of nerve sheath which closes to form brain caudally and spinal cord rostrally. The closure should occur at around the 28^th day of conception failing which the brain or spinal cord does not form properly.

Numerous types of neural tube defect are recognized including ^5,6:

• myelomeningocele (50%)

• anencephaly (40%)

• encephalocele (5%)

• craniorachischisis

• tethered cord

• exencephaly 

• iniencephaly (rare)

• meningoencephalocele

• meningocele

Epidemiology

Neural tube defects affect approximately >1-11 out of 1000 pregnancies ^4-6. 

Pathology

Etiology

Folic acid deficiency is one of the leading causes of anencephaly. By supplementing with folic acid in early pregnancy, this defect can be prevented. It is more prevalent in developing countries and in mothers of low socioeconomic status.

Associations

• maternal diabetes

• maternal obesity ⁷

• antiseizure medication use in pregnancy ⁸

Markers

• elevated maternal serum alpha-fetoprotein (MSAFP) levels 

• elevated amniotic fluid acetylcholinesterase (AChE) levels: in an open neural tube defect ³

Radiographic features

These are different for each entity and best discussed under each subtype.

Diagnosis

Most neural tube defects can be diagnosed by one of the following tests:

• maternal serum alpha-fetoprotein (MSAFP): a screening test performed in the pregnant woman serum during 16-18 weeks of pregnancy (elevated)

• amniocentesis: invasive procedure, performed during 15 weeks of pregnancy

• antenatal ultrasound: allows detection of anencephaly/acrania at 12 weeks of pregnancy

  • absence of intracranial translucency can be seen in open spina bifida ⁹

Treatment and prognosis

Both the management and prognosis is heavily dependent on the type of neural tube defect. The risk for a subsequent pregnancy is thought to be ~5-10%.